U.S. Pat. No. 3,948,215 is an example of a large number of references showing belt type fusers in which a toner image carried on a receiving sheet is positioned against a heated endless belt. The belt heats the toner to its glass transition temperature. A pair of rollers applies pressure between the belt and the receiving sheet to fix the toner image to the receiving sheet. After fixing, the toner image is allowed to remain in contact with the belt until it is cooled below its glass transition temperature. At this point, it is separated. See also, European Patent Application 0301585, published Feb. 1, 1989 and European Patent Application 0295901.
U.S. Patent Application 405,258, filed Sep. 11, 1989, in the name of Rimai et al and U.S. Pat. No. 5,023,038, issued June 11, 1991 in the name of Aslam et al, disclose a method and apparatus for fixing a multicolor toner image carried on a heat-softenable outside layer of a receiving sheet. The receiving sheet is preheated to raise the temperature of the thermoplastic layer to its softening point. It is fed into a pressure nip created by a pressure roller in a belt or web backed by a heated roller. The belt or web is a hard ferrotyping material such as stainless steel, nickel or the like. Relatively high pressure is applied between the belt and pressure roller to imbed much or all of the toner image in the thermoplastic layer, fixing the image. The hard ferrotyping belt, combined with the thermoplastic layer, provides photographic quality with an absence of relief and a high gloss. The image and heat-softenable layer remain in contact with the belt as it moves away from the pressure nip. The belt and receiving sheet are allowed to cool until the heat-softenable layer is below its glass transition temperature. At this point, the receiving sheet is separated without offset and without the use of offset-preventing substances like powders or liquids which would reduce the photographic quality of the image.
In designing a continuous production image-forming apparatus, the ferrotyping surface is formed on a web. The web is usually in the form of an endless belt but it is also known to be quite long and to have supply and take-up rolls for continuous operation.
A problem in using a web system, especially an endless belt system in a productive image-forming apparatus is associated with the time required for the belt and image to cool while maintained in contact. If the fixing device is moved at a speed below the speed of the transfer station to allow cooling, then the mismatch of speeds between the transfer station and the fixing device must be accommodated. In general, this requires either a full frame distance in the in-track direction between the transfer station or drum and the fixing device, or a loop or other mechanism absorbing the difference in speeds. Cooling is the critical time-consuming activity that forces these accommodations.
A number of references suggest actively cooling the belt and receiving sheet combination to reduce the necessary size of the belt required. U.S. Pat. Nos. 4,780,742 and 3,948,215 suggest air cooling the belt and receiving sheet after it leaves the pressure-applying members. See also, U.S. Pat. Nos. 5,012,291; 3,356,831; 3,948,215; "Belt Fusing Device", Research Disclosure, July 1990, page 559; and U.S. Pat. application Ser. No. 07/754,489, filed Sep. 3, 1991 to J. P. Swapceinski et al.
U.S. Patent application Ser. No. 783,475 to Johnson and Merle, entitled "IMAGE-FORMING APPARATUS INCLUDING TONER IMAGE FIXING DEVICE USING FUSING SHEETS", filed Oct. 28, 1991, suggests using a finite or a cut fusing sheet instead of an endless belt, which fusing sheet can form a sandwich with the receiving sheet, which can be moved much slower or not at all during the cooling process after leaving a pair of pressure members that can be allowed to run at full machine speed.